In this application, the term "document" is used to refer to any valuable sheet of paper or other body having a magnetic layer such as ink containing a magnetic pigment having specified magnetic properties.
There exists a need for a magnetic assembly for detection and authentication of magnetic documents which is simple, which has high intrinsic sensitivity, which has a signal output with high signal-to-noise ratio and with independence from the speed of documents passed by the assembly, and which has minimal power dissipation and heating. Because the magnetic qualities of magnetic ink pigment is likely to be poor, with low coercivity and low square hysteresis, it is desirable to have the ability to make in-field measurements of the documents, i.e., to read their characteristic magnetic signal with an MR sensor element while the document is being simultaneously magnetized with an external magnetic field sufficiently large to partially or fully saturate the magnetic pigment in the ink of the document. It is also desirable that such a magnetic assembly reduce the reverse magnetic field seen by a document either before or after passing the MR sensor element.
A need also exists for a magnetic assembly which is capable of sensing the magnetic field in a magnetic layer of a document simultaneously with its production. U.S. Pat. No. 3,796,859, issued Mar. 12, 1974, inventor Thompson, discloses a magnetic ink recording system including a U-shaped permanent magnet recording head for magnetizing magnetic ink on a document. A magnetoresistive (MR) sensing element is located within the yoke of the permanent magnet. The MR sensing element both is biased by the permanent magnet and senses the magnetized ink simultaneously with its production. The system disclosed in this patent is disadvantageous because the yoke ends of the permanent magnet are located close to the magnetized ink of a document and tend to demagnetize the ink before it can be sensed by a magnetic read assembly downstream from the magnetic head. Moreover, the U-shaped permanent magnet can be expensive and difficult to machine for a given application.